1. Field of the Invention
The present invention relates generally to water purification systems for general industrial usage and, more particularly, to a reverse osmosis recovery systems that incorporate recoveries in excess of 85% or utilizing a secondary recovery on the concentrate.
2. Description of the Related Art
Virtually all large municipal water supply systems must treat their water in accordance with an extensive regime of global, federal and state agency regulations. Generally, water treatment occurs before the product reaches the consumer, and then again afterwards (when it is discharged). Water purification usually occurs close to the final delivery points to reduce pumping costs and the chances of the water becoming contaminated after treatment.
Traditional surface water treatment plants generally consists of three steps: clarification, filtration and disinfection. Clarification refers to the separation of particles (dirt, organic matter, etc.) from the water stream. Chemical addition (i.e. alum, ferric chloride) destabilizes the particle charges and prepares them for clarification either by settling or floating out of the water stream. Sand, anthracite or activated carbon filters refine the water stream, removing smaller particulate matter. While other methods of disinfection exist, the preferred method is via chlorine addition. Chlorine effectively kills bacteria and most viruses and maintains a residual to protect the water supply through the supply network.
Increasingly in recent years, water conservation has placed additional pressures on municipal water systems. The goals of water conservation efforts include: Sustainability; Energy Conservation; and Habitat Conservation. Sustainability is generally identified as limiting the withdrawal of fresh water from an ecosystem to a limit that does not exceed its natural replacement rate. Energy conservation is achieved through decreased need for water pumping, delivery, and wastewater treatment. In some regions of the world water purification and treatment consumes a significant amount of energy, (such as, for example, California, where over 15% of total electricity consumption is devoted to water management). Habitat conservation is achieved through minimizing human water use in order to help preserve fresh water habitats for local wildlife and migrating waterfowl.
While the goals of water conservation efforts at a macro level have justifiable economic benefits, in relation to a single municipal water system the result of water conservation efforts can often inevitably lead to increased per unit cost of production. The development and operation of fixed infrastructure over a small demand footprint can lead to, and has led to, significant increases in the cost of purified municipal water and the cost of wastewater sewer charges. While changes in such cost structures may not extremely effect individual residential users, industrial water users see significant effects to the operation of their facilities. Aside from just the cost of the commodity or services, which can double or triple in cost within a short period, limitations placed on an overall water balance within an aquifer can lead to limits on intake and discharge. While the residential user can merely forgo watering the lawn or washing the car, in such situations the industrial user can face production cuts, or limitations in industrial output. In such scenarios, the only way to increase output would be to increase efficiency of processing (or decrease of waste product).
Increased efficiency of water processing can result in a solution that may be effective to a specific process or process stream. Example include saltwater desalination, brine recovery in mine reclamation, or similar situations having specific product or process train requirements. However, such solutions tend to stay site specific and do not share a generally applicable benefit. Developing a general method for improving water purification efficiency must taking into account that water can containing a variety of hardness compounds such as barium, calcium, magnesium, iron, silica, carbonate and bicarbonate, fluoride and sulfate. And, variations in hardness can be commonly found in surface water supplies such as lakes and rivers as well as underground water supplies such as water wells and aquifers and as aqueous industrial effluents and landfill leachates.
Such water is frequently purified by using water softeners in the form of “ion exchange resins”, chemical softeners using the cold lime or hot lime softening process, reverse osmosis and nanofiltration membranes and/or distillation. Most industrial users need purified water containing low to very low concentrations of hardness compounds and of soluble inorganic compounds in order to supply their cooling towers, low-pressure and high pressure boilers, heat exchangers and various process uses. The pharmaceutical and electronics industry users, as well as hospitals and laboratories, require high purity waters which are almost completely free from inorganic compounds. The water purification processes listed above involve transferring the soluble water impurities to a resin bed which must be regenerated and/or disposed of at high cost. Further, adding a large quantity of chemicals can generate a considerable volume of chemical waste in the case of lime softening. In the case of state-of-the-art RO and NF membrane processes, usage of reverse osmosis (RO) or nanofiltration (NF) membranes generates substantial volumes of concentrates which must be treated further or disposed of at a large cost. And, in the case of distillation, very high capital and/or operating costs exist.
Although membrane filtration processes such as reverse osmosis (RO) or nanofiltration (NF) have provided an effective and economically viable means for purifying water, these membrane processes in their current form are limited in the percentage of purified water produced, known as permeate or product recovery. Reverse Osmosis utilizes a thin film composite membrane to remove dissolved salts from a feed water source. Since most of the soluble compounds are separated and concentrated into a smaller volume, typically 25-50% (and sometimes as much as up to 75%) of the volume of the original water source becomes permeate. Water passes through the membrane, while most of the dissolved salts do not pass through the membrane. As such, the membrane concentrate volume is too large and costly to dispose of, except in seawater desalination where the concentrate stream (also known as the reject stream) is returned to sea and in some other applications where there are no regulatory limits on the quantity of the reject stream discharged or the concentration of inorganic compounds contained therein.
Additionally, the main reason why further recovery of purified water from RO or NF membranes is not possible is the tendency of scale to form on the surface of the membranes as the concentration of scale-forming compounds and sparingly soluble salts is increased beyond their saturation values. This deposition of scale frequently results in a loss of purified water production (also known as loss of permeate flux through the membrane) and the eventual need for costly replacement of the membranes.
Typically, Reverse Osmosis (RO) systems operate as a cross-flow filter were a portion of the feed water passes through the RO membrane (typically 75%) and a portion is discharged as a wastewater (25%). The feed water is pressurized (typically below the rating of standard pressure vessels, between 100-600 psig, depending on backpressure) (P-1) to provide the force required to drive the water through the RO membrane. The driving force required to produce a given volume of permeate is dependent upon the feed water salt concentration and water temperature. After passing through the membrane, permeate is typically at fairly low pressure 10-100 psig (P-3), while the concentrate remains at much higher pressure typically 100-600 psig (P-2). A control valve (V-1) is utilized to adjust the concentrate flow and also reduce the concentrate pressure suitable for discharge.
A disadvantage of the reverse osmosis process is the recovery is typically limited to 60-80% as calculated by Equation 1:Percent Recovery=[(Feed water−Permeate)×100]/Feed water  [Eq. 1]As the cost of city water and wastewater disposal increases, minimizing the feed water and concentrate volume is of interest to many RO system operators. The use of chemical additives in the water supply such as acids to reduce the pH and inorganic or organic anti-scalant compounds is practiced in the water treatment and membrane industry in order to provide some improvement in the water recovery and prevent scale formation. However, such improvement is only of limited extent since no anti-scalant is effective for all the contaminants and therefore they do not provide economically viable options for treatment of the entire water stream.
A search of the prior art for a solution to the problem did not disclose any patents that read directly on the claims of the instant invention. However, in the parent application U.S. Ser. No. 12/964,874 by the present inventors and incorporated by reference herein as if fully rewritten, an apparatus and method is provided for maintaining reverse osmosis recovery in excess of 70-80% on a continue basis and without the need for additional energy input (i.e. though additional pump-supplied or pressure) between various stages. While such an apparatus and method provide for an improved water purification system for general industrial usage and may be incorporated in or overlap with the present invention, it has been found that additional adaptations, modifications and improvements, but still based upon such core technology, are preferable extensions under specialized conditions outside the median of most general industrial usages or under such conditions where optimization of such a high efficiency water purification system for specific operational variables is desired.